Something like evolving virtual creatures would be a fun (although pretty ambitious) addition [1].

There's a processing 2D virutal creatures model called sticky feet [2] (source available) that perhaps could be ported to js pretty quickly.

[1] https://www.youtube.com/watch?v=JBgG_VSP7f8

[2] http://www.cc.gatech.edu/~turk/stickyfeet/